In a variety of technologies that utilize chemicals, in order to recycle or reuse the chemicals one must eliminate water and/or other associated impurities. For example, a number of industries utilize large volumes of concentrated inorganic acids as cleaning or etching agents.
This is particularly true in the manufacturing processes of electronic and semiconductor devices. There, parts, such as integrated circuit wafers and printed circuit boards, must be washed in concentrated acid in either an etching step or a cleaning step. One specific example is the pre-diffusion cleaning and photoresist stripping that is accomplished in the manufacture of integrated circuit wafers through the use of concentrated sulfuric acid and accompanying oxidants, for instance, an aqueous solution of hydrogen peroxide.
Inevitably, therefore, the acid baths used for the above purposes become contaminated, such as from the materials cleaned or etched, mixed; or become mixed with the water carried by the oxidant, and/or absorb water from various sources. Most of the contaminants, other than water, have proven relatively simple to remove from the acids. Unlike water, such contaminants may be filtered, precipitated and filtered, or driven off from the acid through use of fairly moderate heat. However, because acids are in general hydrophilic, it has heretofore been difficult to regenerate and purify acids to the concentration that is necessary to reuse the acids in the given industrial process.
Consequently, there has been a tendency for industries to generate enormous quantities of acid wastes. This is neither economical nor is it environmentally sound. Thus, a need exists for small scale, on-site equipment for reprocessing of such chemicals. Indeed, the art has recognized this need and there has been increased attention to the development of methods and apparatus to recycle and purify chemical wastes, such as acids. E.g., U.S. Pat. Nos. 4,828,660 and 4,855,023.
In these aforementioned patents, the oxidizing agents and other contaminants were first stripped from the used contaminated acids, with the combination of heating the mixture and purging the mixture with air or an inert gas stream to remove the water. However, this process is fairly slow and not particularly efficient. Nevertheless, a interesting finding arose in this art, that the resulting recycled concentrated acids may, in many cases, be purer than the commercially purchased starting acid.
In co-pending patent applications, Ser. Nos. 07/648,543 and 07/783,632, now U.S. Pat. Nos. 5,225,048 and 5,236,555, respectively a method and an apparatus are disclosed, respectively, to concentrate liquids through a distillation process utilizing constant, predetermined parameters. Such parameters include, water content and flow rate of the feed, the distillate flow rate, and the heating power for the distillation column. These copending applications are directed primarily at situations in which the chemical sought to be purified is more volatile than water. More specifically, they are directed toward reprocessing single or multiple-phase organic solvents that are more volatile than water.
In the case of concentrated acids, and also many other chemical compounds, the chemical compound sought to be purified is less volatile than water. In such situations, many problems not encountered with more volatile chemicals will become evident to one skilled in the art.
Thus, there remains a need in the chemical reprocessing and purification art for an apparatus and method to more efficiently reprocess and purify chemical compounds, wherein the chemical compound sought to be purified is less volatile than its contaminants. Accordingly, it is one object of the present invention to provide a method for the concentration, separation, and purification of such chemical compounds, and, in particular, water-contaminated liquids, such as inorganic acids.
Another object of the present invention is to provide a novel apparatus for the concentration, separation, and purification of such chemical compounds, in particular the as applied to used inorganic acids.